Technical Field
The present disclosure relates to a micromechanical detection structure of a MEMS (Micro-Electro-Mechanical Systems) multi-axis gyroscope, which has reduced drifts of the corresponding electrical parameters in the presence of thermal deformations, or stresses of various nature acting from outside on a corresponding package.
Description of the Related Art
In particular, the following discussion will make explicit reference, without this implying any loss of generality, to a biaxial MEMS gyroscope having a sensing-mass arrangement subjected to sensing movements along a vertical axis z, i.e., in a direction orthogonal to a horizontal plane of main extension and to the top surface of a corresponding substrate (in addition, possibly, to being able to perform further sensing movements in the same horizontal plane).
It is known that micromechanical detection structures of MEMS sensors with vertical axis z are generally subject to drift of electrical parameters, due to the deformations of a corresponding substrate of semiconductor material, on account, for example, of thermal phenomena, mechanical stresses of various nature acting from outside on the package of the same sensors (for example, due to soldering to a printed-circuit board), or swelling due to humidity.
FIG. 1 shows a micromechanical structure 1 of a known type, of a MEMS sensor with vertical axis z (which further comprises an electronic reading interface, not illustrated, electrically coupled to the same micromechanical structure).
The micromechanical structure 1 comprises: a substrate 2 (including semiconductor material, for example, silicon); and a sensing mass 3, which is of conductive material, for example, polysilicon, and is arranged above the substrate 2, suspended at a certain distance from its top surface.
The sensing mass 3 has a main extension in a horizontal plane xy, which is defined by a first horizontal axis x and a second horizontal axis y that are mutually orthogonal and is substantially parallel to the top surface of the substrate 2 (in a resting condition, in the absence, that is, of external quantities acting on the micromechanical structure 1), and a substantially lower dimension along the vertical axis z, which is perpendicular to the aforesaid horizontal plane xy and which forms with the first and second horizontal axes x, y a set of three Cartesian axes xyz.
The sensing mass 3 has, at the center, a through opening 4, which traverses it throughout its thickness. This through opening 4 has in top plan view a substantially rectangular shape, which extends in length along the first horizontal axis x and is set at the centroid (or center of gravity) O of the sensing mass 3. The through opening 4 consequently divides the sensing mass 3 into a first portion 3a and a second portion 3b, arranged on opposite sides with respect to the same through opening 4 along the second horizontal axis y.
As illustrated schematically also in FIG. 2a, the micromechanical structure 1 further comprises a first fixed electrode 5a and a second fixed electrode 5b, of conductive material, which are arranged on the top surface of the substrate 2, on opposite sides with respect to the through opening 4 along the second horizontal axis y, to be positioned, respectively, underneath the first and second portions 3a, 3b of the sensing mass 3, at a respective distance of separation (or gap) Δz1, Δz2 (which, in resting conditions, have substantially the same value).
The first and second fixed electrodes 5a, 5b define, together with the sensing mass 3, a first sensing capacitor and a second sensing capacitor with plane and parallel faces, which are designated as a whole by C1, C2 and have a given value of capacitance at rest.
The sensing mass 3 is anchored to the substrate 2 by a central anchorage element 6, constituted by a pillar element, which extends within the through opening 4 starting from the top surface of the substrate 2, centrally with respect to the through opening 4. The central anchorage element 6 corresponds to the only point of constraint of the sensing mass 3 to the substrate 2.
In particular, the sensing mass 3 is mechanically connected to the central anchorage element 6 by a first elastic anchorage element 8a and a second elastic anchorage element 8b, which extend within the through opening 4, aligned, with a substantially rectilinear extension, along an axis of rotation A parallel to the first horizontal axis x, on opposite sides with respect to the central anchorage element 6. The elastic anchorage elements 8a, 8b are configured to be compliant to torsion about their direction of extension, thus enabling rotation of the sensing mass 3 out of the horizontal plane xy (in response to an external quantity to be detected, for example, an acceleration or an angular velocity).
Due to rotation, the sensing mass 3 approaches one of the two fixed electrodes 5a, 5b (for example, the first fixed electrode 5a) and correspondingly moves away from the other of the two fixed electrodes 5a, 5b (for example, from the second fixed electrode 5b), generating opposite capacitive variations of the sensing capacitors C1, C2.
Suitable interface electronics (not illustrated in FIG. 1) of the MEMS sensor, electrically coupled to the micromechanical structure 1, receives at input the capacitive variations of the sensing capacitors C1, C2, and processes these capacitive variations in a differential manner for determining the value of the external quantity to be detected.
The present Applicant has realized that the micromechanical structure 1 described previously may be subject to measurement errors in case the substrate 2 undergoes deformation.
The package of a MEMS sensor is in fact subject to deformation as the temperature varies, these deformation due to the different coefficients of thermal expansion of the materials of which it is made, causing corresponding deformations of the substrate 2 of the micromechanical structure 1 contained therein. Similar deformations may further occur on account of particular stresses induced from outside, for example, during soldering of the package on a printed-circuit board, or else on account of phenomena of swelling due to humidity.
As illustrated schematically in FIG. 2b, due to the deformations of the substrate 2, the fixed electrodes 5a, 5b, which are directly constrained thereto (these electrodes are in general set on the top surface of the substrate 2), follow the same deformations of the substrate, while the sensing mass 3 moves, following the displacements of the central anchorage element 6.
Deformation of the substrate 2 may cause both a variation, or drift, of static offset (at time zero) or of the so-called output in response to a zero input (ZRL—Zero-Rate Level), i.e., of the value supplied at output in the absence of quantities to be detected (for example, in the absence of an angular velocity acting from the outside), and a variation of sensitivity in the detection of quantities.
In the example illustrated, the substrate 2 and the corresponding top surface undergo a deformation along the vertical axis z with respect to the second horizontal axis y (in the example, a bending), and, due to this deformation, variations occur in the average distances (or gaps) Δz1 and Δz2 that separate the sensing mass 3 from the substrate 2 at the first and second fixed electrodes 5a, 5b. 
The aforesaid variations of distance cause corresponding variations of the capacitance of the sensing capacitors C1, C2 that are not linked to the quantity to be detected and thus cause undesired variations of the sensing performance of the micromechanical structure 1.
In particular, in the case where the deformation of the substrate 2 causes substantially equal variations of the gaps Δz1 and Δz2, a variation of the sensitivity of detection of the micromechanical structure 1 occurs (understood as ratio ΔC/Δz). In the case of differential variation of the gaps Δz1 and Δz2, a capacitive offset at time zero occurs and/or a variation of the ZRL during operation of the MEMS sensor.
To overcome the above drawbacks, solutions have been proposed, designed in general to eliminate, or at least reduce, the effects of the deformations of the substrate 2 on the micromechanical structure 1.
For instance, document No. US 2011/0023604 A1, filed in the name of the present Applicant, describes a micromechanical detection structure for a MEMS inertial accelerometer with a single sensing axis (the vertical axis z), which has reduced drifts.
In brief, this solution basically envisages anchorage of the sensing mass of the micromechanical structure at anchorages (or points of constraint to the substrate) set in the proximity of the fixed electrodes. In this way, deformations of the substrate affect in a substantially similar way the position of the fixed electrodes and the arrangement of the sensing mass, minimizing the effects of these deformations.
The solution described in the aforesaid document US 2011/0023604 A1 refers, however, only to a micromechanical structure of a uniaxial inertial accelerometer. In particular, there is no reference to how the solution described may be adopted in more complex detection structures, such as, for example, that of a multi-axis MEMS gyroscope in which it is necessary to co-ordinate multiple driving and sensing movements, without altering the characteristics of the same movements.
In a known way, MEMS gyroscopes operate on the principle of relative accelerations, exploiting Coriolis acceleration. When an angular velocity is applied to a moving mass of a corresponding micromechanical detection structure, which is driven in a linear direction, the mobile mass “feels” an apparent force, or Coriolis force, which causes a displacement thereof in a direction perpendicular to the linear driving direction and to the axis about which the angular velocity is applied. The mobile mass is supported above a substrate via elastic elements that enable driving thereof in the driving direction and displacement in the direction of the apparent force, which is directly proportional to the angular velocity and may, for example, be detected via a capacitive transduction system.